An R-T-B based sintered magnet (where R is composed of a light rare-earth element RL and a heavy rare-earth element RH, in which RL is Nd and/or Pr, and RH is at least one element of Dy, Tb, Gd, and Ho; and T is a transition metal element, indispensably containing Fe) that contains an R2T14B type compound as a main phase is known as a magnet with the highest performance among permanent magnets. This type of magnet is used in various motors for hybrid automobiles, electric automobiles, home appliances, etc.
In particular, when used in motors for hybrid automobiles or electric automobiles, the R-T-B based sintered magnet is required to have high coercivity HcJ (hereinafter sometimes simply referred to as “HcJ”). Conventionally, to improve the HcJ, a heavy rare-earth element (mainly Dy) is added in a large amount to this kind of magnet.
However, the heavy rare-earth elements, especially Dy, have various issues, including inconsistent supply and large fluctuations in price due to minimal abundance and restricted areas where their resources are located, and the like. For this reason, users have recently requested improvement of HcJ in R-T-B based sintered magnets while reducing the use of heavy rare-earth elements, such as Dy, as much as possible with no reduction in Br.
Patent Documents 1 to 3 have proposed that in the R-T-B based sintered magnet, Ga or the like is added while setting a B content lower than the general B content (that is, lower than the B content in a stoichiometric proportion of R2T14B type compound), thereby achieving the high HcJ with suppression of degradation in Br while reducing the use of heavy rare-earth elements, such as Dy, as much as possible.
Patent Document 1 mentions that the B content is set lower than that in the standard R-T-B based alloy. Simultaneously, at least one metal element selected from Al, Ga, and Cu is contained to form an R2T17 phase, thus ensuring an adequate volume ratio of a transition-metal-rich phase (R6T13M) that is generated using the R2T17 phase as a raw material. In this way, an R-T-B based rare-earth sintered magnet with high coercivity can be obtained.
Patent Document 2 mentions that an alloy containing Co, Cu, and Ga, yet possessing a boron content lower than the critical boron content in a conventional R-T-B based permanent magnet, exhibits a high coercive force HcJ at the same residual magnetization Br, compared to the conventional alloy.
Patent Document 3 mentions that the contents of B, Al, Cu, Co, Ga, C, and O are set within respective predetermined ranges, while the B content is set lower than that of a standard R-T-B based alloy. Further, an atomic ratio of Nd and Pr to B as well as an atomic ratio of Ga and C to B are respectively set to satisfy specific relationships, whereby high residual flux density and coercivity can be achieved.    Patent Document 1: WO 2013/008756 A    Patent Document 2: JP-2003-510467 W    Patent Document 3: WO 2013/191276 A